U.S. patent application number 10/673610 was filed with the patent office on 2004-04-15 for lithium secondary battery and method for manufacturing thereof.
Invention is credited to Fukui, Atsushi, Kusumoto, Yasuyuki, Minami, Hiroshi, Sayama, Katsunobu, Tarui, Hisaki, Torimae, Mariko, Yagi, Hiromasa.
Application Number | 20040072067 10/673610 |
Document ID | / |
Family ID | 32072461 |
Filed Date | 2004-04-15 |
United States Patent
Application |
20040072067 |
Kind Code |
A1 |
Minami, Hiroshi ; et
al. |
April 15, 2004 |
Lithium secondary battery and method for manufacturing thereof
Abstract
A lithium secondary battery comprising an electrode in which an
active material layer which includes an active material that
electrochemically occludes and releases lithium is formed on a
current collector, wherein cracks are formed in the active material
layer by occlusion and release of lithium ions and thereafter a
solid electrolyte is formed in the cracks in the active material
layer.
Inventors: |
Minami, Hiroshi; (Kobe-city,
JP) ; Sayama, Katsunobu; (Kobe-city, JP) ;
Yagi, Hiromasa; (Nishinomiya-city, JP) ; Fukui,
Atsushi; (Kobe-city, JP) ; Torimae, Mariko;
(Kobe-city, JP) ; Kusumoto, Yasuyuki; (Kobe-city,
JP) ; Tarui, Hisaki; (Kobe-city, JP) |
Correspondence
Address: |
KUBOVCIK & KUBOVCIK
SUITE 710
900 17TH STREET NW
WASHINGTON
DC
20006
|
Family ID: |
32072461 |
Appl. No.: |
10/673610 |
Filed: |
September 30, 2003 |
Current U.S.
Class: |
429/212 ;
29/623.1; 29/623.5; 429/217; 429/245; 429/303; 429/304 |
Current CPC
Class: |
Y02E 60/10 20130101;
Y10T 29/49115 20150115; H01M 4/386 20130101; H01M 4/40 20130101;
H01M 10/052 20130101; H01M 4/0419 20130101; H01M 4/38 20130101;
Y10T 29/49108 20150115; H01M 2300/0085 20130101; H01M 10/0565
20130101; H01M 4/661 20130101; H01M 10/058 20130101; H01M 50/46
20210101; H01M 4/1395 20130101; H01M 4/622 20130101; Y02P 70/50
20151101; H01M 4/134 20130101; H01M 4/0404 20130101 |
Class at
Publication: |
429/212 ;
429/303; 429/304; 429/245; 429/217; 029/623.1; 029/623.5 |
International
Class: |
H01M 004/62; H01M
010/40; H01M 004/66; H01M 004/04; H01M 010/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 2002 |
JP |
2002-285741 |
Feb 5, 2003 |
JP |
2003-027805 |
Claims
What is claimed is:
1. A lithium secondary battery comprising an electrode and a
nonaqueous electrolyte, the electrode comprising an active material
layer provided on a current collector and containing an active
material which is capable of electrochemically occluding and
releasing lithium and having cracks formed in the layer by
occlusion and releasing of lithium, the cracks of the active
material layer being filled with the nonaqueous electrolyte in the
form of a solid electrolyte.
2. The lithium secondary battery according to claim 1, wherein the
entirety of the nonaqueous electrolyte is the solid
electrolyte.
3. The lithium secondary battery according to claim 1, wherein the
nonaqueous electrolyte partially comprises the solid
electrolyte.
4. The lithium secondary battery according to claim 1, wherein the
solid electrolyte is a gel polymer electrolyte comprising a polymer
and an electrolyte containing a lithium salt.
5. The lithium secondary battery according to claim 4, wherein the
polymer is a polyether solid polymer, polycarbonate solid polymer,
polyacrylonitrile solid polymer, copolymers of at least two of
these polymers or crosslinked polymers thereof.
6. The lithium secondary battery according to claim 1, wherein a
surface roughness (Ra) of a surface of the current collector is at
least 0.2 .mu.m.
7. The lithium secondary battery according to claim 1, wherein the
current collector is a copper foil, a copper alloy foil or a metal
foil having a copper layer or a copper alloy layer on a surface
thereof.
8. The lithium secondary battery according to claim 1, wherein the
current collector is an electrolytic copper foil, an electrolytic
copper alloy foil or a metal foil having an electrolytic copper
layer or an electrolytic copper alloy layer on a surface
thereof.
9. The lithium secondary battery according to claim 1, wherein the
active material layer is formed by sintering, under a non-oxidizing
atmosphere, a slurry comprising particles of the active material
and a binder applied on the surface of the current collector.
10. The lithium secondary battery according to claim 9, wherein the
binder remains after sintering.
11. The lithium secondary battery according to claim 9, wherein the
binder is a polyimide.
12. The lithium secondary battery according to claim 9, wherein the
mean diameter of the active material particles is 10 .mu.m or
less.
13. The lithium secondary battery according to claim 9, wherein an
electrically-conductive powder is mixed in the slurry, and the
electrically-conductive powder is included in the active material
layer.
14. The lithium secondary battery according to claim 9, wherein the
active material layer is formed by coating the slurry on the
current collector, drying the slurry, rolling the dried slurry and
then sintering.
15. The lithium secondary battery according to claim 1, wherein the
active material layer is deposited on the current collector as a
thin film.
16. The lithium secondary battery according to claim 1, wherein the
active material is silicon, tin, germanium, aluminum, or an alloy
containing these elements.
17. A method for manufacturing a lithium secondary battery
comprising a nonaqueous electrolyte and an electrode on which an
active material layer containing an active material capable of
electrochemically occluding and releasing lithium is formed on a
current collector, wherein cracks which are formed in the active
material layer by occlusion and release of lithium are filled with
a solid electrolyte, comprising: preparing a temporary-battery
comprising the electrode and the electrolyte comprising a lithium
salt; forming cracks in the active material layer by charging and
discharging the temporary-battery; adding a polymerizable monomer
to the electrolyte in the temporary-battery and polymerizing the
monomer to form the solid electrolyte and to fill the cracks with
the solid electrolyte.
18. A lithium secondary battery comprising an electrode and a
nonaqueous electrolyte, the electrode comprising an active material
layer formed on a current collector by deposition of an active
material which is capable of electrochemically occluding and
releasing lithium and having cracks formed in the layer by
occlusion and releasing of lithium, the cracks of the active
material layer being filled with the nonaqueous electrolyte in the
form of a solid electrolyte.
19. A method for manufacturing a lithium secondary battery
comprising a nonaqueous electrolyte and an electrode on which an
active material layer containing an active material capable of
electrochemically occluding and releasing lithium is formed as a
thin film on a current collector, wherein cracks which are formed
in the active material layer by occlusion and release of lithium
are filled with a solid electrolyte, comprising: preparing a
temporary-battery comprising the electrode in which an active
material layer containing an active material capable of
electrochemically occluding and releasing lithium is formed by
depositing a thin film of the active material on a current
collector, and the electrolyte comprising a lithium salt; forming
cracks in the active material layer by charging and discharging the
temporary-battery; adding a polymerizable monomer to the
electrolyte in the temporary-battery and polymerizing the monomer
to form the solid electrolyte and to fill the cracks with the solid
electrolyte.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a lithium secondary battery
and a method for manufacturing the lithium secondary battery.
BACKGROUND OF THE INVENTION
[0002] A lithium secondary battery that comprises a nonaqueous
electrolyte and utilizes the transfer of lithium ions between a
positive electrode and a negative electrode for charge and
discharge of the battery has recently been used as one of new type
high output and high energy density batteries.
[0003] A lithium secondary battery using a material which forms an
alloy with lithium as a negative electrode active material is
known. However, it is also known that an active material which
forms an alloy with lithium increases and decreases in volume when
lithium ions are occluded and released, and the active material is
pulverized during charge and discharge cycles and separates from
the current collector. This causes deterioration of current
collecting characteristics (current collectability) and of charge
and discharge cycle characteristics.
[0004] A negative electrode for a lithium secondary battery in
which an active material layer comprising a silicon material and a
binder is formed on a current collector comprising an
electrically-conductive metal foil and is sintered on the current
collector under a non-oxidizing atmosphere has been proposed
(Japanese Patent application No. 2000-401501). The negative
electrode provides excellent charge and discharge cycle
characteristics.
[0005] It has also been found that when a thin amorphous or micro
crystalline silicon film which is provided on a current collector
comprising an electrically-conductive metal foil by sputtering
method or CVD method is used as a negative electrode active
material, excellent charge and discharge cycle characteristics are
obtained (International Publication No. 01/31720).
[0006] However, a negative electrode increases and decreases in
volume when an active material occludes and releases lithium ions,
cracks occur in the active material layer, and contact resistance
in the active material layer increases. This causes deterioration
of current collecting characteristics (current collectability) and
of charge and discharge cycle characteristics.
OBJECT OF THE INVENTION
[0007] An object of the present invention is to provide a lithium
secondary battery and a method for preparing the lithium secondary
battery which is capable of improving current collectability of an
electrode and improving charge and discharge cycle
characteristics.
SUMMARY OF THE INVENTION
[0008] The present invention relates to a lithium secondary battery
comprising a nonaqueous electrolyte and an electrode in which an
active material layer which includes an active material that
electrochemically occludes and releases lithium is formed on a
current collector, and wherein cracks in the active material layer
are filled with the nonaqueous electrolyte in the form of a solid
electrolyte.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a cross section of the negative electrode of an
embodiment of the present invention.
[0010] FIG. 2 is a plan view of the lithium secondary battery
prepared in the Experiments.
[0011] FIG. 3 is a photograph taken by a scanning electron
microscope from the top showing the condition of the negative
electrode having cracks after charging and discharging.
[0012] FIG. 4 is a photograph taken by a scanning electron
microscope showing the condition of a cross section of the negative
electrode having cracks after charging and discharging.
[0013] FIG. 5 is a cross section of the electrode of another
embodiment of the present invention.
[0014] FIG. 6 is a graph showing charge and discharge
characteristics of an embodiment of the present invention.
[0015] FIG. 7 is a photograph taken by a scanning electron
microscope showing the condition of a cross section of the
electrode having cracks in the thin film after charging and
discharging.
[0016] FIG. 8 is a graph showing charge and discharge
characteristics of a battery of the present invention.
EXPLANATION OF ELEMENTS
[0017] 1: current collector
[0018] 1a: surface of current collector
[0019] 2: active material layer
[0020] 2a: active material particles
[0021] 2b: binder
[0022] 3: solid electrolyte
[0023] 4: cracks
[0024] 5: thin film
[0025] 6: cracks
DETAILED EXPLANATION OF THE INVENTION
[0026] When a solid electrolyte is provided in the cracks formed in
the active material layer, current collectability of the electrode
is improved and charge and discharge cycle characteristics are also
improved. The solid electrolyte prevents the active material layer
from separating from the current collector. It is also helpful in
improving charge and discharge cycle characteristics.
[0027] In the present invention, the nonaqueous electrolyte can be
a solid electrolyte. That is, the solid electrolyte provided in the
cracks can be a part of a solid nonaqueous electrolyte. The
nonaqueous electrolyte can also partially include the solid
electrolyte. For example, the solid electrolyte can be provided
only in the cracks, and the remainder of the nonaqueous electrolyte
can be liquid.
[0028] As the solid electrolyte, there can be a solid electrolyte
in which a polymer and an electrolyte containing a lithium salt are
combined to make a gel. That is, a gel polymer in which the polymer
supports the electrolyte containing a lithium salt can be
illustrated. The solid electrolyte including a gel polymer has
excellent adherence to the active material, and the adherence to
the active material is not deteriorated by charging and
discharging. Therefore, charge and discharge cycle characteristics
can be significantly improved. As the polymer, polyether solid
polymer, polycarbonate solid polymer, polyacrylonitrile solid
polymer, copolymers thereof and crosslinked polymers can be
illustrated.
[0029] As other polymers, fluoropolymer, for example,
polyvinylidene fluoride, polyvinylidene
fluoride--hexafluoropropylene copolymer, polytetrafluoroethylene,
and the like, polyamide polymer, polyimide polymer, polyimidazole
polymer, polyoxazole polymer, polymelamine-formaldehyde polymer,
polypropylene polymer, polysiloxane polymer, and the like, can be
illustrated.
[0030] The solid electrolyte of the present invention can be an
entirely solid electrolyte having lithium ion conductivity. As the
entirely solid electrolyte, an electrolyte comprising a lithium
salt and a polymer can be illustrated. As the polymer for the
entirely solid electrolyte, polyether polymer, polysiloxane
polymer, and polyphosphazene polymer can be illustrated.
[0031] The surface of the current collector onto which the active
material layer is provided preferably has a surface roughness (Ra)
of at least 0.2 .mu.m. A current collector having such surface
roughness can provide a sufficient contact area for the active
material layer and the current collector to improve the adhesion of
the active material layer and the current collector. When a binder
is included in the active material layer, the binder penetrates
into the uneven surface of the current collector, and an anchor
effect occurs between the binder and the current collector to
increase adhesion. Therefore, peeling of the active material layer
from the current collector can be prevented. When active material
layers are provided on both surfaces of the current collector, both
surfaces preferably have a surface roughness of at least 0.2
.mu.m.
[0032] The surface roughness (Ra) and the average distance (S)
between adjacent local peaks preferably satisfy the relationship
100 Ra.gtoreq.S. The surface roughness (Ra) and the average
distance (S) between adjacent local peaks are defined in the
Japanese Industrial Standards (JIS B 0601-1994), and can be
measured by a surface roughness tester.
[0033] A current collector treated to have a roughened surface can
be used in the electrode of the present invention. As a method of
roughening the surface, plating, vapor phase epitaxy, etching,
polishing and the like can be illustrated. Plating and vapor phase
epitaxy are methods for forming an uneven layer on the surface of
the current collector. Plating can be electrolytic or
non-electrolytic. As vapor phase epitaxy, there can be illustrated
sputtering, chemical vapor deposition (CVD), evaporation, and the
like. As etching, physical or chemical etching can be used. As
polishing, there can be illustrated polishing with sand paper,
blasting, and the like.
[0034] As the current collector for the present invention, an
electrically conductive metal foil is preferable. As examples of
such metal foil, a conductive metal foil composed of a metal such
as copper, nickel, iron, titanium, cobalt and the like, and an
alloy containing any combination thereof can be illustrated. The
current collector preferably contains a metal element that easily
diffuses into the particles of the active material. From this point
of view, when the active material is silicon or the like, a metal
foil containing copper is preferred because copper is easily
diffused into silicon. Therefore, a copper metal foil and a copper
alloy foil are more preferred.
[0035] It is also possible to use a metal foil having a layer
containing copper on a surface as the current collector to improve
adherence of the current collector and the active material layer.
That is, a copper or copper alloy layer provided on the surface of
a metal foil which does not include copper can be used. As the
metal foil having a layer containing copper having a surface
roughness (Ra) of at least 0.2 .mu.m, a copper or copper alloy is
provided by electrolytic plating on the metal foil. Concretely, an
electrolytic copper foil on which a copper or copper alloy plating
is formed by electrolytic plating, a nickel foil plated with copper
or a copper alloy can be illustrated.
[0036] There is no limitation with respect to the thickness of the
current collector (Y). However, a thickness of 10.about.100 .mu.m
is preferable. There is no limitation regarding the upper limit of
the surface roughness (Ra) of the surface of the current collector.
However, the upper limit is preferably not greater than 10 .mu.m
because the thickness of the current collector (Y) is preferably in
a range of 10.about.100 .mu.m.
[0037] The thickness (X) of the active material layer preferably
satisfies relationships with the thickness (Y) of the current
collector and the surface roughness (Ra) of the current collector
of 5Y.gtoreq.X, and 250Ra.gtoreq.X. If such relationships are
satisfied, deformation, for example, wrinkles, and the like, of the
current collector can be prevented, and the active material layer
can be prevented from peeling off of the current collector.
[0038] A thickness of the active material layer (X) of not greater
than 100 .mu.m is preferred, and a thickness in a range of
10.about.100 .mu.m is more preferred. The active material layer of
the present invention can be active material particles adhered by a
binder on the current collector, or can be a thin film deposited on
the current collector.
[0039] When the active material layer comprises the active material
particles and binder, the active material layer is formed by
sintering under a non-oxidizing atmosphere after the active
material layer is provided on the surface of the current collector.
The binder preferably does not completely decompose after the heat
treatment for sintering. If the binder remains after the heat
treatment and is not decomposed, the binding ability of the binder,
as well as sintering, increases adhesion between particles of the
active material and between the active material and the current
collector. If an electrically conductive metal foil having a
surface roughness (Ra) of at least 0.2 .mu.m is used as the current
collector, the binder penetrates into the uneven surface of the
current collector, and an anchor effect occurs between the binder
and the current collector to increase adhesion. Even if the volume
of the active material increases or decreases during occluding and
releasing of lithium ions, peeling of the active material layer
from the current collector can be prevented and excellent charge
and discharge cycle characteristics can be obtained.
[0040] As the binder, a binder containing polyimide is preferred.
Polyimide can be obtained by heat treatment of polyamic acid.
Polyimide is obtained by heat treatment of polyamic acid by
dehydration condensation to form polyimide. A yield of imide of the
polyimide is preferably at least 80%. If the yield of imide of the
polyimide is less than 80%, adherence of the active material
particles and the current collecter may be deteriorated. The yield
of imide means the mol % of the produced polyimide to the polyimide
precursor (polyamic acid). Polyimide having an imide yield of at
least 80% can be obtained when polyamic acid in
N-methyl-2-pyrrolidone (NMP) is heated at 100.about.400.degree. C.
for not less than one hour. If the temperature is 350.degree. C.,
the imide yield is 80% for about a one hour heat treatment, and is
100% for about a three hour heat treatment. It is preferred in this
invention that the binder is not completely decomposed after heat
treatment for sintering. Therefore, if polyimide is used as the
binder, it is preferred that the heat treatment for sintering is
done at a temperature of not greater than 600.degree. C.
[0041] An amount of the binder in the active material layer is
preferably at least 5% based on the total weight of the active
material layer. A volume of the binder is preferably at least 5% of
the total volume of the active material layer. If the amount of the
binder in the active material layer is too little, the binder may
not be able to provide sufficient adhesion in the electrode. If the
amount of the binder in the active material layer is excessive,
resistance in the electrode increases to make the initial charge
difficult. Therefore, the amount of the binder in the active
material layer is preferably not greater than 50 weight % of the
total weight of the layer, and the volume of the binder in the
active material layer is preferably not greater than 50% of the
total volume of the layer.
[0042] There are no limitations with respect to the negative
electrode active material if a material is capable of occluding and
releasing lithium. A material which forms an alloy with lithium is
preferably used. As such material, silicon, germanium, tin, lead,
zinc, magnesium, sodium, aluminum, gallium, indium, and alloys
thereof can be illustrated. Especially, silicon, tin, germanium,
aluminum, and an alloy thereof are preferable. Silicon is
especially preferred because it has a large theoretical capacity.
As the silicon alloy, a solid solution of silicon and at least one
additional element, an intermetallic compound of silicon and at
least one additional element, a eutectic alloy of silicon and at
least one additional element, and the like can be illustrated. The
alloy can be prepared by arc melting, liquid quenching, mechanical
alloying, sputtering, chemical vapor deposition, calcining, or the
like. As liquid quench, single roll quenching, double roll
quenching, atomizing, for example, gas atomizing, water atomizing,
disc atomizing, and the like, can be illustrated.
[0043] Particles of the active material coated with a metal or the
like can also be used. The particles can be coated by electroless
plating, electrolytic plating, chemical reduction, vapor
deposition, sputtering, chemical vapor deposition, or the like. As
the metal used to coat the surface of the particles, it is
preferred to use the same metal as used for the electrically
conductive metal foil. If the particles are coated with the same
metal as the metal foil, the degree of bonding with the current
collector dramatically improves, and excellent charge and discharge
cycle characteristics can be obtained.
[0044] There are no limitations with respect to the mean diameter
of particles of the active material. However, the mean diameter is
preferably not greater than 100 .mu.m, and more preferably, not
greater than 50 .mu.m, and further preferably, not greater than 10
.mu.m. As the diameter of the active material particle is smaller,
better cycle characteristics are obtained.
[0045] An electrically conductive powder can be mixed in the active
material layer. The active material layer containing the
electrically conductive powder can be formed by mixing the
electrically conductive powder in a slurry of active material
particles and binder. If an electrically conductive powder is mixed
in the layer, an electrically conductive network is formed around
the particles of the active material to increase current
collectability of the electrode. As the electrically conductive
powder, materials similar to the electrically conductive metal foil
can preferably be used. Concretely, copper, nickel, iron, titanium,
cobalt and the like, and an alloy or a mixture of these elements
can preferably be used alone or in combination thereof. Copper
powder is preferable as a metal powder. An electrically conductive
carbon powder can also preferably be used.
[0046] As with the active material particles, there are no
limitations with respect to the mean diameter of particles of the
electrically conductive powder. However, the mean diameter is
preferably not greater than 100 .mu.m, and more preferably, not
greater than 50 .mu.m, and further preferably, not greater than 10
.mu.m.
[0047] Sintering under a non-oxidizing atmosphere can be performed
under, for example, a nitrogen atmosphere, an inert gas atmosphere
(for example, argon or the like), and the like. It is also possible
to perform the sintering under a reducing atmosphere, for example,
a hydrogen atmosphere, or the like. The temperature used for the
sintering is preferably lower than the melting point of the current
collector and of the particles of the active material. For example,
when a copper foil is used as the current collector, it is
preferred that the sintering temperature is not greater than the
melting point of copper, i.e., 1083.degree. C. The temperature used
for sintering is preferably in a range of 200.about.500.degree. C.,
and more preferably, in a range of 300.about.400.degree. C. As a
method of sintering, spark plasma sintering, hot pressing, or the
like, can be used.
[0048] In the present invention, preferably after the active
material layer is provided on the current collector and prior to
sintering, the active material layer with the underlying current
collector is subject to rolling. Rolling can increase packing
density in the active material layer and adhesion between particles
of the active material and between the active material and the
current collector to improve charge and discharge cycle
characteristics.
[0049] As described above, the active material layer can be formed
by depositing the active material in the form of a thin film on the
current collector. The thin film of the active material can be
formed by sputtering, chemical vapor deposition (CVD), evaporation,
spray coating, electroless plating, electrolytic plating, and the
like. As the active material to form the thin film, silicon, tin,
germanium, aluminum, and an alloy containing these elements, and
the like, can be illustrated. Silicon is most preferably used. When
silicon is used, it is used in a form of an amorphous and micro
crystalline silicon film.
[0050] As the solid electrolyte in the present invention, a gel
electrolyte which is prepared from a polymer and an electrolyte
including a lithium salt is preferred as described above. There is
no limitation with respect to the solvent to be used for the
nonaqueous electrolyte. Cyclic carbonates, for example, ethylene
carbonate, propylene carbonate, butylene carbonate, and the like;
chain carbonates, for example, dimethyl carbonate, methylethyl
carbonate, diethyl carbonate, and the like, can be used alone or in
combinations thereof. A mixture of the cyclic carbonate described
above and an ether, for example, 1,4-dioxane, 1,2-dimethoxyethane,
1,2-diethoxyethane, and the like, can also be used.
[0051] As a lithium salt, LiPF.sub.6, LiBF.sub.4,
LiCF.sub.3SO.sub.3, LiN(CF.sub.3SO.sub.2).sub.2,
LiN(C.sub.2F.sub.5SO.sub.2).sub.2, LiN (CF.sub.3SO.sub.2)
(C.sub.4F.sub.9SO.sub.2), LiC (CF.sub.3SO.sub.2).sub.3- ,
LIC(C.sub.2F.sub.5SO.sub.2).sub.3, LiAsF.sub.6, LiClO.sub.4,
Li.sub.2B.sub.10C.sub.10, Li.sub.2B.sub.12Cl.sub.12, and the like,
can be used alone or in various combinations thereof. A mixture of
LiXF.sub.y (where X is P, As, Sb, B, Bi, Al, Ga or In; when X is P,
As or Sb, y is 6; and when X is Bi, Al, Ga or In, y is 4) and
lithium perfluoroalkylsulfonylimide, LiN(C.sub.mF.sub.2m+1SO.sub.2)
(C.sub.nF.sub.2n+1SO.sub.2) (where m and n are each independently
an integer of 1.about.4), or lithium perfluoroalkylsulfonylmethide,
LiC (C.sub.pF.sub.2p+1SO.sub.2) (C.sub.qF.sub.2q+1SO.sub.2)
(C.sub.rF.sub.2r+1SO.sub.2) (where p, q and r are each
independently an integer of 1.about.4) can preferably be used.
Especially, a mixture of LiPF.sub.6 and
LiN(C.sub.2F.sub.5SO.sub.2).sub.2 is preferred.
[0052] As the positive electrode active material for the lithium
secondary battery, lithium-containing transition metal oxides, for
example, LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2O.sub.4, LiMnO.sub.2,
LiCo.sub.0.5Ni.sub.0.5O.sub.2,
LiNi.sub.0.7Co.sub.0.2Mn.sub.0.1O.sub.2, and the like, and metal
oxides not containing lithium, for example, MnO.sub.2, and the
like, can be illustrated. In addition to the materials described
above, if the material is electrochemically capable of occluding
and releasing lithium, the material for the positive electrode is
not limited.
[0053] As the solid electrolyte, a combination of an electrolyte,
including a lithium salt, and polymer and which is gelatinized is
preferably used. It is preferable that a monomer of the polymer is
added into the electrolyte to gelatinize the electrolyte by
polymerization of the monomer.
[0054] In a method for manufacturing the lithium secondary battery
of the present invention, a negative electrode, a positive
electrode and an electrolyte including a lithium salt are housed in
a container to prepare a temporary-battery, and the
temporary-battery is charged and discharged to create cracks in the
active material layer, and then a monomer of a polymer is added to
the electrolyte to polymerize the monomer and to gelatinize the
electrolyte to prepare a solid electrolyte.
[0055] Stated differently, the method for manufacturing the lithium
secondary battery of the present invention is a method to prepare
the battery including a negative and positive electrode wherein an
active material layer including an active material
electrochemically capable of occluding and releasing lithium is
formed on a current collector of an electrode and cracks formed in
the active material layer created by occluding and releasing
lithium are filled with the solid electrolyte prepared from the
polymer and electrolyte. The method comprises preparing a
temporary-battery comprising the negative electrode, positive
electrode and electrolyte in the container, wherein the electrolyte
includes a lithium salt, forming cracks in the active material
layer by charging and discharging the temporary-battery, and
forming in the cracks, the solid electrolyte that is prepared by
gelatinizing the electrolyte by polymerization of the monomer added
to the electrolyte in the temporary-battery after formation of the
cracks.
[0056] According to the method of the present invention, the
temporary-battery containing the electrolyte before gelatinization
of the electrolyte is charged and discharged to form cracks in the
active material layer of the negative electrode. The electrolyte
penetrates into the cracks formed in the active material layer. The
monomer is added into the electrolyte and is polymerized to form
the solid electrolyte by gelatinizating the electrolyte. Therefore,
the solid electrolyte can be easily filled in the cracks in the
active material layer.
[0057] FIG. 1 is a cross section of a negative electrode of the
present invention comprising an active material layer comprising
active material particles and a binder. As shown in FIG. 1, a
surface 1a of a current collector 1 is unevenly formed, and an
active material layer 2 is formed on the current collector. The
active material layer 2 comprises active material particles 2a and
binder 2b, and includes cracks 4 in a direction of thickness. The
cracks 4 are formed by the occlusion and release of lithium from
the active material particles 2a. A solid electrolyte 3 is
penetrated into the cracks 4. Each part of the active material
layer 2 separated by the cracks is covered by the solid electrolyte
3.
[0058] The solid electrolyte 3 having lithium ion conductivity
fills the cracks 4 of the active material layer 2 to prevent the
active material layer 2 from peeling off or separating from the
current collector 1, and improves charge and discharge cycle
characteristics.
[0059] The active material layer 2 is held together by the
mechanical strength of the solid electrolyte 3. Therefore, the
active material layer 2 is prevented from coming off the current
collector 1, and charge and discharge cycle characteristics are
improved.
[0060] According to another aspect of the present invention, at
least one of the active material layers of the positive electrode
and the negative electrode is formed by deposition of the active
material as a thin film on the current collector, and the solid
electrolyte is filled in cracks formed in the active material layer
by occluding and releasing lithium.
[0061] A lithium secondary battery according to the another aspect
of the invention includes positive and negative electrodes, in
which an active material layer includes an active material
electrochemically capable of occluding and releasing lithium, and a
nonaqueous electrolyte, wherein at least one of the active material
layers of the positive and negative electrodes is formed as a thin
film on the current collector by deposition of an active material,
and cracks formed in the active material layer by occluding and
releasing lithium are filled with a solid electrolyte.
[0062] The lithium secondary battery of the other aspect of the
present invention can be manufactured by preparing a
temporary-battery comprising the positive electrode, the negative
electrode and the electrolyte comprising a lithium salt; forming
cracks in the active material layer by charging and discharging the
temporary-battery; adding a monomer of a polymer to the electrolyte
in the temporary-battery; and then polymerizing the monomer to
prepare the solid electrolyte by gelatinization of the electrolyte
and to form the solid electrolyte in the cracks.
[0063] FIG. 5 is a cross section of an electrode of a second aspect
of the present invention. As shown in FIG. 5, a surface 1a of the
current collector 1 is unevenly formed. A thin film 5 as an active
material layer is formed on the uneven surface of the current
collector. Cracks 6 are formed in a direction of thickness of the
thin film 5. The cracks 6 are formed by occluding and releasing
lithium from the thin film 5. The thin film 5 was continuous before
the cracks were formed and the surface of the thin film was uneven
corresponding to the surface of the current collector. The thin
film 5 increases and decreases in volume when lithium ions are
occluded and released, and stress generated by change of volume
creates cracks 6 starting from a valley of the surface of the thin
film toward a direction of the thickness. The thin film 5 has a
pillar structure divided by cracks 6.
[0064] As shown in FIG. 5, the solid electrolyte is filled into the
cracks 6. Each portion of the thin film 5 divided by the cracks is
covered by the solid electrolyte. Therefore, current collectability
of the electrode can be improved because the solid electrolyte
having lithium ion conductivity fills the cracks 6 of the thin film
5.
[0065] The thin film 5 is held together by the mechanical strength
of the solid electrolyte 3. Therefore, the thin film 5 is prevented
from coming off the current collector 1, and charge and discharge
cycle characteristics are improved.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0066] Embodiments of the present invention are explained in detail
below. It is of course understood that the present invention is not
limited to these embodiments and can be modified within the spirit
and scope of the appended claims.
Experiment 1
Preparation of Negative Electrode
[0067] 81.8 parts by weight of the silicon powder (purity 99.9%)
was added to 8.6 weight % of a N-methyl-2-pyrrolidone solution
containing 18.2 weight parts of polyimide as a binder and the
components were mixed and kneaded by a pestle in a mortar to
prepare a negative electrode mixture slurry.
[0068] The slurry was coated on one surface of an electrolytic
copper foil (thickness: 35 .mu.m) having a surface roughness (Ra)
of 0.5 .mu.m which is a current collector, and was rolled after
drying. The coated copper foil was sintered by heating at
400.degree. C. for 30 hours under an argon atmosphere to prepare a
negative electrode. The thickness of the electrode (including the
current collector) was 50 .mu.m. Therefore, the thickness of the
active material layer was 15 .mu.m. Thickness of the active
material layer (X)/surface roughness of the copper foil (Ra) was
30. Thickness of the active material layer (X)/thickness of the
copper foil (Y) was 0.43.
[0069] In the negative electrode, the density of the polyimide was
1.1 g/cm.sup.3, and the volume of polyimide was 31.8% based on the
total volume of the active material layer including polyimide.
Preparation of Positive Electrode
[0070] Li.sub.2CO.sub.3 and COCO.sub.3 were measured to an atomic
ratio of 1:1 and were mixed in a mortar. The mixture was pressed in
a mold having a diameter of 17 mm, and was sintered at 800.degree.
C. for 24 hours in air to obtain sintered LiCoO.sub.2. It was
ground in a mortar to particles having a mean diameter of 20
.mu.m.
[0071] 90 parts by weight of the LiCoO.sub.2 powder and 5 parts by
weight of artificial carbon powder as a electrically conductive
agent were mixed with 5 weight % of N-methyl-2-pyrrolidone solution
containing 5 parts by weight of polyfluorovinylidene as a binder to
prepare a positive electrode mixture slurry. The slurry was coated
on aluminum foil which was a current collector, and was rolled
after drying to prepare a positive electrode.
Preparation of Electrolyte
[0072] 1 mol/l LiPF.sub.6 was dissolved in a mixture (3:7) of
ethylene carbonate and diethylene carbonate and 5 weight % of
vinylene carbonate was added to prepare an electrolyte.
Preparation of Pregel Solution
[0073] Tripropylene glycol diacrylate (molecular weight 300) and
the electrolyte were mixed in a ratio by mass of 1:7, and 5000 ppm
of t-hexyl peroxy pivalate as a polymerization initiator was added
to the mixture to prepare a pregel solution.
Assembly of Battery
[0074] The positive and negative electrodes with positive and
negative electrode current collecting tabs mounted thereon and a
separator made of porous polyethylene were rolled and placed in an
outer battery can made of an aluminum laminate to prepare a
temporary-battery having an outer measurement of 35 mm in width, 50
mm in length and a thickness of 3.5 mm. The temporary-battery was
charged to 4.2 V at a current of 50 mA, and then was discharged to
2.75 V at a current of 50 mA. Then the same weight of the pregel
solution as the electrolyte in the temporary-battery was added into
the battery and the solution and the electrolyte were mixed and
left for four hours to provide a uniform mixture. The battery was
heated at 60.degree. C. for three hours to gelatinize the mixture
to prepare a battery A1. The polymerizable compound (monomer) in
the pregel solution, tripropylene glycol diacrylate, was
polymerized by heating of the mixture, and the electrolyte was held
in a mesh structure of the polymer to form a so-called gel polymer
solid electrolyte.
[0075] FIG. 2 is a plan view of the lithium secondary battery
prepared above. The lithium secondary battery is sealed by heat
sealing of outer edge of the outer battery can 11 made of an
aluminum laminate to form sealed opening 12. The positive electrode
current collecting tab 13 and the negative electrode current
collecting tab 14 are mounted on an upper part of the outer battery
can 11. The set of electrodes separated by the separator made of
porous polyethylene is inserted in the outer battery can 11.
Observation of Negative Electrode After Charge and Discharge of
Temporary-Battery
[0076] FIGS. 3 and 4 are photographs taken by a scanning electron
microscope showing the condition of the negative electrode after
charging and discharging of the temporary-battery. As is clear from
FIGS. 3 and 4, a crack in a direction of thickness of the active
material layer was formed by charging and discharging of the
temporary-battery. In this example, the pregel solution was added
after the cracks were formed and then the gel polymer solid
electrolyte was formed. Therefore, the gel polymer solid
electrolyte formed to fill the cracks.
Experiment 2
[0077] Battery B1 was prepared in the same manner as in Experiment
1 except that the temporary-battery was not charged and
discharged.
[0078] Battery B2 was prepared in the same manner as in Experiment
1 except that an electrolyte without a monomer and a polymerization
initiator was used instead of the pregel solution.
[0079] Charge and discharge cycle characteristics of batteries A1,
B1 and B2 were evaluated. Each battery was charged to 4.2 V at a
current of 100 mA and 25.degree. C., and then was discharged to
2.75 V at a current of 100 mA and 25.degree. C. (this is considered
to be one charge and discharge cycle). The number of cycles to
reach 80% of the discharge capacity of the first cycle was measured
to determine the cycle life of the battery. The results are shown
in Table 1. The cycle life of each battery is shown as an index
when the cycle life of the battery A1 is taken as 100.
1 TABLE 1 Charge/Discharge of Pregel Battery Temporary-battery
Solution Cycle Life A1 Yes Yes 100 B1 No Yes 23 B2 Yes No 69
[0080] In battery A1, the temporary battery was charged and
discharged to form cracks in the active material layer, and then
the electrolyte was gelatinized. In contrast, the solid electrolyte
was formed before battery B1 was charged and discharged, and cracks
in the active material layer were formed after the solid
electrolyte was formed. Therefore, the solid electrolyte did not
penetrate into the cracks.
[0081] The battery B2 was charged and discharged after the
temporary-battery was assembled, but the pregel solution was not
used. The electrolyte was not gelatinized, i.e., the electrolyte
was in a normal liquid condition.
[0082] As is clear the results shown in Table 1, battery Al of the
present invention had a longer cycle life in comparison with
battery B1. It is believed that the solid electrolyte filled the
cracks in the active material layer, current collectability of the
electrode was increased and the active material was efficiently
used. The solid electrolyte in the cracks held the active material
layer together to prevent the active material layer from separating
from the current collector and to improve charge and discharge
cycle characteristics.
Experiment 3
[0083] The effect of surface roughness (Ra) of the current
collector was evaluated.
[0084] Batteries A2 and A3 were prepared in the same manner as
Experiment 1 except that electrolytic copper foils having a surface
roughness (Ra) of 0.2 .mu.m and 0.17 .mu.m, respectively, were used
instead of the electrolytic copper foil having a surface roughness
(Ra) of 0.5 .mu.m.
[0085] Cycle characteristics of batteries A2 and A3 were evaluated
in the same manner described above. Cycle life is described as an
index when the cycle life of battery A1 is taken as 100. Table 2
also includes the cycle life of battery A1.
2 TABLE 2 Roughness of Surface of Current Collector Battery (.mu.m)
Cycle Life A1 0.5 100 A2 0.2 87 A3 0.17 76
[0086] As is clear from the results shown in Table 2, batteries A1
and A2 prepared using a current collector having a surface
roughness (Ra) of at least 0.2 .mu.m have excellent cycle
characteristics as compared to battery A3 prepared using a current
collector having a surface roughness (Ra) of less than 0.2 .mu.m.
It is believed that the contact area of the particles of the active
material and the surface of the current collector is increased by
using a metal foil having a surface roughness (Ra) of at least 0.2
.mu.m. Additionally, sintering effectively increases adhesion of
the particles of the active material and the current collector, and
the binder penetrates into uneven portions of the surface of the
current collector, and the adhesion increases because of an anchor
effect occurring in the binder and the current collector to
increase current collectability of the electrode.
Experiment 4
[0087] The effect of sintering conditions of the electrodes on
cycle characteristics was evaluated. Battery A4 was prepared in the
same manner as Experiment 1 except that the electrode was treated
at 550.degree. C. for ten hours. Battery B3 was prepared in the
same manner as Experiment 1 except that the electrode was not
treated by heat.
[0088] Cycle characteristics of batteries A4 and B3 were evaluated
in the same manner as described above. Cycle life is described as
an index when the cycle life of battery A1 is taken as 100. Table 3
also includes the cycle life of battery A1.
3 TABLE 3 Battery Heat Treatment Condition of Electrode Cycle Life
A1 400.degree. C., 30 hrs 100 A4 550.degree. C., 10 hrs 75 B3 None
32
[0089] As is clear from the results shown in Table 3, batteries A1
and A4 have excellent cycle characteristics as compared to battery
B3 prepared without heat treatment of the electrode. It is believed
that the particles of the active material and the current collector
were sintered by heat treatment and adhesion of the active material
layer and current collector increased to improve the current
collectability of the electrode.
[0090] Battery A4 in which the electrode is treated at 550.degree.
C. for ten hours reduced the cycle characteristics as compared to
battery Al in which the electrode is treated at 400.degree. C. for
30 hours. It appears that the binder was decomposed by the heat
treatment at 550.degree. C., and adhesion resulting from the binder
in the electrode was significantly reduced to decrease the current
collectability.
Experiment 5
[0091] The effect of an electrically conductive powder added to the
active material layer was evaluated.
[0092] Battery A5 was prepared in the same manner as Experiment 1
except that 20 weight % (based on the weight of the copper powder
and the silicon powder) of copper powder of a mean diameter of 3
.mu.m was added to the silicon powder. Cycle characteristics of
battery A5 were evaluated in the same manner as described above.
Cycle life is described as an index when the cycle life of battery
A1 is taken as 100. Table 4 also includes the cycle life of battery
A1.
4 TABLE 4 Battery Electrically-Conductive Powder Cycle Life A1 None
100 A5 Copper Powder 103
[0093] As is clear from the results shown in Table 4, battery A5 in
which copper powder is added to the active material had better
cycle characteristics than battery A1 which did not include
electrically conductive powder in the active material. The
electrically conductive powder is believed to have formed a network
around the particles of active material to improve the current
collectability in the active material layer.
Experiment 6
Preparation of Negative Electrode
[0094] Copper was deposited by electrolysis on a surface of a
rolled copper film of a thickness of 18 .mu.m to prepare a copper
film having a roughened surface (thickness of 26 .mu.m, surface
roughness Ra of 0.21 .mu.m). An amorphous silicon thin film was
deposited by sputtering to a thickness of 5 .mu.m. Direct current
pulse was used as power for sputtering. Conditions of sputtering
are as follows:
5 Frequency of direct current pulse: 100 kHz Width of direct
current pulse: 1856 ns Power of direct current pulse: 2000 W Argon
flow rate: 60 sccm Pressure of Gas: 2.0.about.2.5 .times. 10.sup.-1
Pa Time: 146 minutes
[0095] The obtained silicon thin film was cut with the current
collector to 25 mm.times.25 mm to prepare a negative electrode.
Preparation of Positive Electrode
[0096] A positive electrode mixture slurry was prepared in the same
manner as Experiment 1. The slurry was coated on aluminum foil
which was a current collector, and was rolled after drying. A 20
mm.times.20 mm piece was cut out from the coated aluminum foil to
prepare a positive electrode.
Preparation of Electrolyte
[0097] An electrolyte was prepared in the same manner as in
Experiment 1.
Preparation of Pregel Solution
[0098] A pregel solution was prepared in the same manner as in
Experiment 1.
Assembly of Battery
[0099] A temporary-battery was prepared in the same manner as in
Experiment 1. The temporary-battery was charged to 4.2 V at a
current of 1.3 mA, and then was discharged to 2.75 V at a current
of 1.3 mA. Then the same weight of the pregel solution as the
electrolyte in the temporary-battery was added into the battery,
the solution and the electrolyte were mixed and left for four hours
to provide a uniform mixture. The battery was heated at 60.degree.
C. for three hours to gelatinize the mixture to prepare a battery
A6. The polymerizable compound (monomer) in the pregel solution,
tripropylene glycol diacrylate, was polymerized by the heating of
the mixture, and the electrolyte was held in a mesh structure of
the polymer to form a so-called a gel polymer solid
electrolyte.
Experiment 7
[0100] A battery B4 was prepared in the same manner as in
Experiment 6 except that the temporary-battery was not charged and
discharged after being assembled.
Evaluation of Charge and Discharge Characteristics
[0101] Charge and discharge cycle characteristics of batteries A6
and B4 were evaluated. Each battery was charged to 4.2 V at a
current of 1.3 mA and 25.degree. C., and then was discharged to
2.75 V at a current of 1.3 mA and 25.degree. C. This is considered
to be one charge and discharge cycle.
[0102] The initial discharge capacity (discharge capacity at the
first cycle) and capacity maintenance rate after ten cycles are
shown in Table 5. After the tenth cycle was completed, discharge
capacity was measured and measurement of capacity maintenance rate
was calculated according to expression (2) below.
Capacity Maintenance Rate (%)=(discharge capacity after ten
cycles/initial discharge capacity).times.100 (2)
[0103] The charge and discharge cycle characteristics are shown in
FIG. 6.
6 TABLE 5 Capacity Charge/ Initial Maintenance Discharge Charge
Rate of Preliminary Capacity after 10 Cycles Battery Battery (mAh)
(%) A6 Yes 11.9 93.3 B4 No 10.6 86.1
[0104] As is clear from FIG. 6 and Table 5, battery A6 of the
present invention has better charge and discharge cycle
characteristics as compared to the comparative battery B4.
Observation of Negative Electrode After Charge and Discharge of
Temporary-Battery
[0105] FIG. 7 is a photograph taken by a scanning electron
microscope showing the condition of the negative electrode after
charging and discharging of the temporary-battery. As is clear from
FIG. 7, cracks in a direction of thickness of the active material
layer were formed by charging and discharging of the
temporary-battery. In this example, the pregel solution was added
after the cracks were formed and then the gel polymer solid
electrolyte was formed. Therefore, the gel polymer solid
electrolyte formed and filled the cracks.
Experiment 8
Preparation of Positive Electrode
[0106] The negative electrode in Experiment 6 was used as a
positive electrode.
Assembly of Battery
[0107] A temporary-battery was prepared in the same manner as in
Experiment 1 except for the use of the positive electrode described
above and the use of a negative electrode made of lithium metal.
Battery A7 was prepared from the temporary-battery in the same
manner as Experiment 6.
Experiment 9
[0108] Battery B5 was prepared in the same manner as in Experiment
8 except that the temporary-battery was not charged and discharged
after being assembled.
Evaluation of Charge and Discharge Characteristics
[0109] Charge and discharge cycle characteristics of batteries A7
and B5 were evaluated. Each battery was charged to 0 V at a current
of 4 mA and 25.degree. C., and then was discharged to 2.0 V at a
current of 4 mA and 25.degree. C. This is considered to be one
charge and discharge cycle. The initial discharge capacity and
capacity maintenance rate after ten cycles are shown in Table 6.
Cycle characteristics during the charge and discharge test are
shown in FIG. 8.
7 TABLE 6 Capacity Charge/ Initial Maintenance Discharge Charge
Rate of Preliminary Capacity after 10 Cycles Battery Battery (mAh)
(%) A7 Yes 14.4 99.3 B5 No 12.5 92.5
[0110] As is clear from FIG. 8 and Table 6, battery A7 of the
present invention has excellent charge and discharge cycle
characteristics.
Advantages of the Invention
[0111] The present invention improves current collectability of an
electrode, and provides a lithium secondary battery having
excellent charge and discharge cycle characteristics.
* * * * *